Moving to SystemC TLM for design and verification of digital hardware

SystemC TLM and HLS are new to many
designers, and successful adoption requires learning new modeling
techniques. Cadence supplies a wide range of HLS design examples with
C-to-Silicon Compiler, which helps designers learn the new techniques
more quickly.

Cadence has developed a library of synthesizable IP
for use with C-to-Silicon Compiler, spanning common building blocks
such as FIFOs, register files, bus interfaces such as AHB and AXI, and
floating- and fixed-point math operations.These models are designed to be highly configurable, and provide good QoR when synthesized to gates.

The
data access controller design described in this paper used the AXI3 TLM
IP and FIFO models provided by Cadence, greatly accelerating the design
process and delivering good QoR. By starting with the examples and
design IP, a designer can quickly get a basic design working, and then
incrementally modify it to meet all design requirements.

The AXI3
TLM IP provided by Cadence enabled Fujitsu Semiconductor’s models to
use a high-level API to access the AXI3 bus, while still providing
access to all protocol features of AXI3. In addition, the AXI3 TLM IP
can be configured to automatically provide a SystemC TLM2 interface to
external models. This makes it possible to use a single model to drive
the HLS flow, as well as for high-speed simulation in TLM2 virtual
platforms, which is something Fujitsu Semiconductor will explore in the
future.

Case Study: A Data Access Controller Design

While
earlier HLS tools focused on datapath-oriented designs, Fujitsu
Semiconductor applied C-to-Silicon Compiler and the AXI3 TLM models to a
control-centric data access controller design. The design is described
in simplified form here, since some aspects are proprietary. It contains
64-bit AXI3 target and initiator interfaces and one to eight logical
channels for data transfer. Figure 1 shows the block diagram of the
design. It uses an AXI3 target interface to configure the registers for
all logical channels for data transfer and an internal FIFO to store the
data from the source address to destination address via the AXI3
initiator interface. The “ChReg” is a set of registers to store the
configuration parameters for each logical channel, and the “CommonRegs”
is a set of registers to store the configuration parameters for all
logical channels.

Figure 1: Data access controller block diagram

Table 1 shows the design parameters.

Table 1: Design Parameters

Fujitsu
Semiconductor implemented the design in SystemC using the Cadence AXI3
TLM IP. To accurately compare with their existing hand-written RTL
design, they implemented the SystemC model as follows:

Use the standard AXI3 protocols with the same parameters as the existing hand-written RTL design.

Implement the same behavior between SystemC and hand-written RTL.

Generate different micro-architectures by the SystemC flow from the hand-written RTL to improve QoR and performance.

Write the SystemC model so that its micro-architecture could be easily reconfigured using C++ parameters.

Figure
2 shows part of the SystemC source code for the design that configures
the AXI3 TLM write address channel of the initiator socket. Line 1
declares the payload of the write address. Lines 2 to 5 set the
attributes of the write address payload with address, transfer length,
burst types, and data size for the AXI3 TLM initiator. Line 6 uses a
while loop to put the write address payload on the write address channel
of the initiator socket “initiator_if.waddr” using a non-blocking
function “nb_put(waddr)”. Note that all of the details of the AXI3
signal-level protocol are hidden in the nb_put() function in the AXI3
TLM library, so that designers can simply use a put/get function call
without having to worry about these details. This significantly improves
the readability of the code and completely separates the behavior from
the interface protocols

In the article it mentions that multiple different types of coverage were "considered" as part of the validation effort. Any results from those considerations? Are there any non-obvious pitfalls or positive results?